A device for measurement of basic sediment and water (BS&W) in a predominantly non-aqueous stream (e.g. pipeline crude oil), is disclosed in U.S. Pat. No. 4,401,575. This device is an improvement on capacitance type instruments of the art which are dependent upon the extent to which the intrinsic dielectric constant of the subject varies with time. The gravity and physical composition of crude oil are two factors which determine its intrinsic dielectric constant. If one or both of these properties should vary, instruments measure the accompanying change in the dielectric constant as percent BS&W. This yields an inaccurate measurement of BS&W because instruments must be initially set to read zero BS&W as the intrinsic dielectric constant of the fluid. The capacitance type instruments of the prior art have no means for automatically correcting the zero BS&W setting to compensate for periodic variations in the oil properties mentioned. By comparison, the device of U.S. Pat. No. 4,401,575 provides for automatic compensation of BS&W measurements by producing a clean, dry sample of the line fluid for measurement of its intrinsic dielectric constant. In this way, the true BS&W content of the fluid is measured by finding the difference between the dielectric constants of the wet and dry streams.
The improvement over the prior art represented by the invention of U.S. Pat. No. 4,401,575 is substantial, and the present invention provides an additional improvement which even further increases the efficiency and accuracy of the invention of U.S. Pat. No. 4,401,575. Thus, it has been observed that the two capacitance cells of the BS&W instrument are identical, but not perfectly so, and therefore display a slight difference in voltage output when containing the same fluid. The cell testing and capacitance of a dry stream appears to build a small increase in its offset from the cell testing capacitance of a wet stream, due to the necessarily low flow rates through the dry stream capacitance cell, which buildup is swept away when the wet stream is run at a higher flow rate through both cells to determine zero offset (the "wet-wet" check). The magnitude of the offset is somewhat uncertain during the periods between wet-wet checks. Accordingly, it is desirable to cancel the zero offset with improved certainty and eliminate the wet-wet checks.